We present the case of a 51 years old woman who saw a 8 cm long cutaneous breast lesion growing slowly in five months without any notion of pain. She had no history of any illness. She was referred to the gynecological surgery unit for its removal.

A benign cutaneous horn was evocated as its base was thin but the diagnostic of a pre invasive intraepithelial dysplasia or squamous cell carcinoma of the skin had to be eliminated by histological examination. It looked like a solid elongated keratinized tumor connected to the skin by a thin bridge of pink/red regular skin (Figure 1 and Figure 2). No evidence of pre invasive skin lesion was found around. The patient had an excision of the lesion and its base under local anesthesia.

Histopathologic examination showed an exophytic lesion, on a flat and neat base, with epidermal hyperplasia and papillomatosis, hyperkeratosis, diskeratosis and parakeratosis (Figure 3). It was associated with a viral cytopathogenic effect and the presence of koïlocytes, as seen in Human Papilloma Virus (HPV) infection (Figure 4). A PCR was made and no evidence of HPV infection was found on the tissue.

The differential diagnosis of a cutaneous horn is only histological l [1]. Four differential diagnoses have to be evocated: hypertrophic actinic keratosis, Bowen's disease, invasive carcinoma [2,3] and hyperkeratotic seborrheic keratosis.

As the lesion showed neither dysplasia nor dermal invasion, no criterion for the three first diagnoses was met. The diagnosis was a hyperkeratotic seborrheic keratosis associated with viral cytopathogenic effect. At 6 month, there is no sign of relapse. This was a rare case of cutaneous horn located to the breast as they generally present on the head or the hands (sun-exposed parts of the body in a context of hypertrophic actinic keratosis). In some 20% of cutaneous horns, malignancy of the base can be found, justifying systematic excision [4]. The skin has to be removed on its whole thickness at the implantation site in order to check for potential malignancy.